Safety

There is serious risk of falling rocks on the coast here. There has been a very large rock fall east of Durdle Door in April 2013, and some smaller ones. See: Dungy Head webpage for details of these. Rock falls can take place from any of the cliffs here, but particularly from high Chalk cliffs; these can be very dangerous. Keep out and away from the foot of the cliffs. Cliffs like those of Hambury Tout and Swyre Head and Bats Head are particularly hazardous. It is sad to record that some years ago a person was killed by a falling rock near Durdle Door. In particular avoid any place where there is a debris cone or freshly fallen rock (i.e. Chalk) on the beach. Avoid climbing to places that are dangerous, including parts of the Durdle Door promontory. Do not approach cliff edges anywhere as there is hazard of overhang or loose top soil and grass.

This webpage is explanatory, geological and does not give guidance to go to any particular place. You do so at your own risk.

INTRODUCTION:

Aerial Photographs

INTRODUCTION:

Geological Maps INTRODUCTION:

Geological Maps

The British Geological Survey map, 1:50,000, Solid and Drift, 2000 Edition, Swanage Sheet, 343 and part of 342, is well worth purchasing. It can be obtained from the British Geological Survey website and is very inexpensive, costing only 12 pounds sterling.

This map is based on Nowell (1997). It is very good and it is recommend that the original paper be consulted for further details. It is particularly useful in not only showing the general geology but also revealing the details of faults in this area. The north-south distance of the Portland Stone from the Chalk varies greatly along this stretch of coast and the reasons are clearer if the details of the fault pattern here is studied.

LOCATION - APPROACH FROM LULWORTH COVE:

Approach from the East

[The path has now been cut by a Chalk rock fall on 29-30th April 2013, and there is a diversion. The steps lead nowhere!]

.

From Lulworth Cove the walk to Durdle Door follows a convenient path obliquely up the side of Hambury Tout. The original chalk path has now been paved with limestone sets, having been subject to serious erosion by feet. On the west side of the hill the steps have been cut by a rock fall, as shown above. Go to: Cliff Collapse at St. Oswald's Bay, part of the Dungy Head website, for more photographs and information.

An alternative means of access to Durdle Door is by driving to the Durdle Door caravan site beyond Hambury Tout, and walking from there.

Here we see the promontory from the cliff-top when following the path over the hill from Lulworth Cove. The outer wall of vertical, Jurassic, Portland Stone (marine oolite) and basal Purbeck Caps (thrombolitic and pelloidal limestones) is obvious. This runs approximately east-west, but with some minor deviations in direction. Notice how the outcrop of the hard stone is shown not only by the outer walls of the promontory but also by outlying rocks. The main part of the lagoonal, Cretaceous, Purbeck Formation is recognised by the thin-bedded limestone and shale. It is not complete because it is truncated here by a fault. The hard Cherty Freshwater Limestone and the Cinder Bed (with oysters) in the Middle Purbeck are conspicuous as a double reef running east west. The Lower Cretaceous Wealden Group is of fluvial sands and clays with some lignite beds. This unit is also vertical and the softer beds have been eroded out by running water so that it is marked by a series of gullies. The grassy hollow to the north (inland direction) of the Wealden has been formed by an old mudslide of the Gault Clay (Upper Cretaceous). I do not know when this occurred but it has bushes within it but no trees. It is thus unlikely to be very old. The Upper Greensand is poorly exposed on this side.

Vertical to overturned Chalk is seen in the cliffs on the right-hand side of this photograph, that is to the north. Notice the interference effect of small waves coming through two gaps in the outer stone barrier. The beach is of subangular brownish granules of flint. It is similar to the beach material on the west side.

It is interesting that the beach material is mainly of flint granules, rather than pebbles as at Lulworth Cove. Is it of about the same particle size on both sides of the promontory in spite of differences in protection from storm waves, differences in fetch and differences in relationship to the prevailing wind.

LOCATION DURDLE DOOR:

The Natural Arch

The western support for the arch of Durdle Door consists both of Portland Freestone, an oolite and some basal Purbeck strata, beneath the Broken Beds, which have been eroded away just here. The details of the Purbecks have not been studied in this inaccessible locality but a distinctive recess would seem to be the Lower Dirt Bed. No Broken Beds are preserved. Thus the northern (right hand side) limestone is probably Hard Cap. It is too thick to be Soft Cap, and that unit may have been eroded away.

Durdle Door Arch and Pillar - more

Notice, incidently the solution effects by the sea on the limestone at the base of the pillar. Take care regarding the cliffs around here. Do not try to cross the arch of Durdle Door. The cliff jumping shown in a picture is clearly not a safe activity, especially since there are submerged rocks at shallow depth in places.

In the past, incidently, there were other risks on this coast. At least one fall from a cliff here was not an accident. On the night of 28th June, 1832, the Chief Officer of the Coastguard, Lieutenant Thomas Knight of Folkstone, was attacked by a band of smugglers near here. He was beaten before being thrown over the cliff near Durdle Door. He died on the following day (Legg, 1989).

The photographs above gives views of Durdle Door and some details from different viewpoints. Both basal Purbeck limestone (with the thrombolites) and the top of the Portland Stone are involved in the wall of Durdle Door. The near-vertical Purbeck limestone is on the north, landward side. The more resistant Portland Stone forms buttresses on the far side against the southwesterly storm waves. The rock immediately beyond is the Bull, of Portland Stone, perhaps with some Purbeck limestone. There was probably once an extension of the stone wall to near this and perhaps an arch which collapsed long ago.

The natural arch of Durdle Door is one of the physical features of the area which has retained a name given to it probably more than a 1000 years ago (Arkell, 1947), showing that the coast does not change very rapidly. The name " Durdle " is derived from an Old English word "thirl", meaning to pierce (as in "nostril"). A similar arched rock in south Devon is known as the Thurlestone. At Durlston Bay, Swanage, there was probably an arched rock of which a stack at Durlston Head (of Purbeck Broken Beds with celestite) is most likely to be the relic. Damon (1884) also mentioned another name: "The most singular feature of this is the natural arch, known as the "Barn-door", formed in the Purbeck Limestone and sufficiently high for a good-sized sailing boat to pass through it."

The shape of the arch is largely controlled by two sets of joints at oblique angles. These are shown in some of the photographs above. Of particular interest are the structures here. The Purbeck Monocline plunges down to the east and here in the west Durdle Door is near the very angular foresyncline, which is in the cliffs westward and beyond Bat's Head. In this vicinity there is overturning which is not seen elsewhere and much strike faulting. Although there has been argument about the direction in which the incompetent strata moved here it could, in fact, only have gone up. There is no room beneath and there is an unconformity between the Jurassic and middle Cretaceous strata.

LOCATION DURDLE DOOR PENINSULA:

The Arete of the Durdle Door Peninsula

The arete of the Durdle Door peninsula is formed by Cypris Freestones, thin-bedded ostracod limestones of the basal Purbeck Formation. They dip steeply northward, and have argillaceous units interbedded. There is a path on the top but it is dangerous away from the central grassy area, and very dangerous out to the far east or west. From the path there is a view down to the south of a steep, irregular limestone cliff, largely of Portland Freestone. To the north the Cypris Freestones continue down with some east-west minor folds.

PORTLAND GROUP

Portland Cherty Series and Portland Freestone

The Portland Stone is not accessible at Durdle Door, except by boat. The Portland Cherty Series is near sea-level on the outer, southern side, with the Portland Freestone forming the upper and larger part of this jagged, rocky slope. The basal Purbeck sequence can only be accessed by scrambling at the eastern promontory, just above sea level, but this is not safe and not recommended (and not actually feasible for older people or field parties).

PURBECK GROUP

Caps and Broken Beds - Stratigraphy

.

The diagrams above are revised from older work of West (1975). See also: West Paper on Purbeck Evaporites -1975 .

.

.

Portland and Basal Purbeck Strata - East End of the Promontory

.

.

.

.

The Purbeck Caps on the eastern side of the promontory are well-displayed in a vertical section. The section does not provide convenient access for non-scramblers. In a recess near the end, the Great Dirt Bed can be seen. The Broken Beds, a little higher stratigraphically, are thinly developed but very obvious. The section is to some extent disturbed by the Broken Beds type of tectonism.

.

Portland and Basal Purbeck Strata - Western End of the Promontory

.

.

On the west side of the promontory access to some of the basal Purbeck strata is not easy. There may be some dificulty in recognising the thin Soft Cap. The Broken Beds though are easily seen there, though.

PURBECK GROUP

Tree Holes and Thrombolites at the Durdle Door Arch

Another "Fossil Forest"

The fairly thick limestone containing tree holes in thrombolites on the western side of Durdle Door seems to be that of the Hard Cap, a fairly thick unit here and not the very thin Soft Cap. There are thrombolites around small tree holes as shown in photographs here. A section was studied and measured at the eastern end in 1960 and the thrombolites there were found to be in the Hard Cap, within a bed about a metre in thickness. They are present above the Lower Dirt Bed. Tree holes were not noticed at that particular exposure, but the easily accessible section at the eastern end is very limited in size.

Incidently, notice in the photograph immediately above there is actually a fifth tree hole, but part of the inner thrombolite sheath has fallen away since 2002 (earlier photograph above).

In the Hard Cap at Durdle Door thrombolites have encrusted trees, but they are much smaller than those in the Soft Cap of the Fossil Forest. They resemble those in the Hard Cap on the Isle of Portland. In a photograph above, a person walking on the arrete gives an approximate scale. Holes left by the rotted trees are clearly seen and at least nine can be recognised. They all seem to be hollow tubes without any fossil wood remains. The moulds are dominantly normal to the bedding, but may be oblique in some case. They were presumably rooted in the Lower Dirt Bed.

The thrombolites around the trees are multilayered, not with the fine lamination of stromatolites but with two or three broad circle. An interesting feature present in two shown here is that there is a thin initial coating, single or double, beneath the main thrombolite material.

A Soft Cap origin of the thrombolites was considered because of the major thrombolite developments in this bed at the Fossil Forest, east of Lulworth Cove. However the bed quite different at Dungy Head, where it is thin and laminated. At the Durdle Door eastern promontory the Soft Cap is 0.34m thick and is coarsely crystalline, not thrombolitic.

The trees in the Hard Cap at this site are small with trunk diameter of only about 25cm. This seems almost like a planted conifer forest of the type common in the New Forest of southern England, with quite close-spaced trees . Evidence elsewhere suggests relatively straight, upright trees of Cypress-type with wood of the genus Cupressinoxylon purbeckensis. See the papers of Jane Francis for more information.

.

PURBECKS AND PORTLAND FREESTONE AT MAN O' WAR ROCKS

Introduction

.

.

.

.

.

Man O' War Rocks and Man O' War Cove are east of the Durdle Door Promontory. The rocks are a continuation of the Portland Stone and the basal Purbeck Caps outcrop from Durdle Door. The Broken Beds are not obvious and have probably been eroded away to below sea level.

A remarkable aspect is that these strata are too close to the Chalk outcrop in the cliffs for the sequence of strata between the basal Purbeck and the Chalk. Some substantial part of the succession must be lost by strike faulting. Because the Portland Stone runs into the unconformity at the base of the Gault and Upper Greensand at depth (to the north) any faulting must have been upwards. This is also true for the strike faulting, presumably similar that is seen in the Purbeck succession in the neck of the Durdle Door promontory. Upward strike faulting of Purbeck strata explains the Lulworth Crumples. They simply accomodate upfaulted Purbeck strata.

.

LOCATION - DURDLE COVE:

Lower Purbeck Group - Cypris Freestones Member and Hard Cockle Member

The Cypris Freestones Member of the Purbeck Group is of high Upper Jurassic age. Above the Cypris Freestones is the Hard Cockle Member. The Jurassic Cretaceous boundary is probably part-way up the Soft Cockle Member. This Member is present at Durdle Door but as not clearly obvious as elsewhere and does not show beds of gypsum as at Worbarrow Tout. The total Purbeck thickness here is only 29.9 metres according to Arkell (1947), but the succession is faulted out a short distance above the Cinder Bed.

.

.

.

Ripple marks are present in the Lower Purbeck Group at Durdle Door. These examples are from the Hard Cockle Member on the west side of the promontory close to the southern end of the beach and vertical limestone wall. You can also see excellent examples in the slightly older Cypris Freestones Member on the east side of the promontory. You can see that they are sharp crested and frequently bifurcate. In one photograph some (not necessarily all) bifurcations have been marked for clarity.

Such ripples conform to the descriptions of wave-formed ripples (oscillation ripples) (Tucker, 1982). These are formed by the action of waves on non-cohesive sediment, usually of medium silt to sand grades. They are typically symmetrical in shape (but not always). The crests of wave-formed ripples are generally straight and bifurcation is common , sometimes rejoining to enclose small depressions. In profile the troughs tend to be more rounded than the crests which can be pointed. The ripple index (wavelength over height) of wave-formed ripples is generally around 6 or 7. If you have time determine the ripple index of these.

The sediment here is a carbonate sand with some quartz. You will not see normal marine fossils. There are no ammonites, belemnites or large, thick-shelled bivalves. Fossils that you are likely to find are the small tests of ostracods (oval objects about 2 mm. in length). They are very abundant at some horizons. Look also for the shells (about 1 centimetre) of the " cockle " Protocardia . Fossil insect may also occur at about this horizon but are not easy to find. Protocardia purbeckensis is very common in the Lower Purbeck Group. It probably occupied an ecological niche in the Purbeck environment much like that which the modern euryhaline (tolerant of various salinities) cockle, Cardium or Cerastoderma sometimes occupies. In Egypt it lives in moderately hypersaline lagoons, although it is dwarfed (West, Ali and Hilmy, 1983). There is evidence in this region for hypersalinity when these Lower Purbeck strata were deposited. Moulds of halite crystals occur at Worbarrow Tout and elsewhere, and in thin-section under the microscope pseudomorphs after gypsum crystals can be found. Although not present at Durdle Door beds of gypsum occur in the Soft Cockle Member above, further east.

The ripples, then, can be seen as evidence of shallow marginal conditions of a hypersaline lagoon, where small waves washed around the sediment. There are sediments casts of halite crystals here too. Dinosaur footprints can sometimes be found in facies like this but they have been seen here.

Higher in the Purbeck Group the Cinder Bed with its numerous small oysters, Praeexogyra distorta, is very easily recognised. It has a bluish grey appearance with numerous cross-sections of small, dark blue oyster shells visible. It had almost completely marine palaeosalinity as shown by the occurrence of echinoids within it Professor Forbes in the 19th Century. Echinoids are stenohaline because they have no osmotic mechanism to survive non-marine salinities They do not usually occur at other horizons within the Purbeck Group of Dorset. This is because salinities were usually above or below those of sea-water, and, in addition, prone to fluctuate rapidly in this extensive shallow lagoon. They probably fluctuated seasonally as well, so that the lagoon became very salty in the dry summer and halite was sometimes deposited from a lagoon that was almost fresh in the winter. One of the best horizons for study of Purbeck freshwater sediments is the Cherty Freshwater Limestone. It is easily found a short distance beneath the Cinder Bed. It is not very good for fossils here because it needs soil weathering and not sea erosion to render the silicified gastropods in relief, rather than just in cross-section.

.

LOCATION - EAST SIDE OF DURDLE DOOR PROMONTORY:

Purbeck Group (Lulworth-Durlston Formations)

.

.

Compare the above diagram, based, but with new labelling, on the situation in about 1936 (Arkell, 1938) to the present visible cliff section. Note particularly that the Hard Cap Limestone are cut out in the lower part of the cliff by strike faulting. This is probably a further point of evidence against the old gravity slip hypothesis for the Lulworth Crumples of Lees (1935). (See more modern work by Underhill and Stoneley, 1998).

.

.

.

.

.

.

On the east side of the connecting ridge of the Durdle Door promontory the Cinder Member and the Cherty Freshwater Limestone Member of the Purbeck Group are conspicuous. Find these first and then move to lesser known horizons from these marker beds. The Cinder Bed, with its small, dark, blue-grey lagoonal oysters, Preexogyra distorta is easily seen as a strong limestone bed but rather bluish and with rounded edges, rather than angular fractures. The Cherty Freshwater Limestone is almost white, hard and splintery with freshwater gastropods and charophytes (not always easy to see on an unleached surface). It does, of course, contain some chert.

LOCATION - DURDLE COVE:

Wealden - General

.

.

Proceeding northward (landward), and stratigraphically above the almost vertical, Purbeck strata, are the soft clays and sands of the Wealden Group. The steps down the cliff are on this. The unit is abnormally thin here but interesting. There is much black, lignitic, fossil wood which has come from conifer trees. It is compressed, friable and pyritic. The organic matter nourished sulphate-reducing bacteria which produced the sulphide ions; reaction with iron in the sediments produced the pyrite and surplus hydrogen sulphide may originally have given an unpleasant odour to the soils of the Wealden marshes and floodplains.

A palaoesol or floodplain soil on the west side of the Durdle Door peninsula has been rendered conspicuous in 2011 by a small landslide. In the photograph above the view is towards the east; the Wealden strata are vertical and younging towards the north (left). The palaeosol shown has reddish and pale green mottling, as is usually the case. The red staining has probably been produced by hematite, resulting from dehydration of the original goethite of the flood-plain gley soil. Oxidised iron minerals usually develop in desiccation cracks and in old roots of such flood plain soils. Away from these locations of access of oxygen the grey or greenish parts may be in reduced condition. Palaeosols like this are common in the Wealden marls in Dorset and in the Isle of Wight (Wessex Formation part of the Wealden Group), and elsewhere, of course.

The Wealden outcrop of Durdle Door is a possible locality for for dinosaur remains. They sometimes occur elsewhere in the Wealden, sometimes with fossil wood. The bones are hard, brown and mineralised. They occur in the Dorset exposures (Swanage and Worbarrow) but are not as frequently found as in the Isle of Wight. I have only found one worn Iguanodon tooth, in the Wealden of Dorset and that was at Swanage. A dinosaur bone has been found in the Wealden at Lulworth Cove.

WEALDEN STRATA

Erosion of the Cliffs

Whereas the Portland and Purbeck limestones are resistant, the soft Wealden strata is being eroded on both sides of the connection ridge from the Chalk high ground to Durdle Door. The Wealden cliff is partially protected on the western side by a substantial accumulation of flint pebbles on the beach from the nearby Chalk cliffs. However, the strongest waves come from the southwest so some direct wave erosion can occur in storms. On the other side of the connection ridge there is less beach material and more protection from storm waves by Man O' War Rocks in addition to the limestone ridge of Durdle Door. However, erosion does also take place here. The connecting ridge is very gradually diminishing in size and this is going to continue with rising sea level and more storms. Eventually of course, the Durdle Door limestone feature will be separated from the shore, and in the long term will become like Man O' War rocks.

.

LOCATION - DURDLE COVE:

The English Channel Inversion - the Boundary

(Margin of the Portland - Isle of Wight Basin)

.

Durdle Cove, directly to the west of the Durdle Door promontory, and the coast beyond has magnificent beaches and cliffs. It is a very educational place in geological terms and has a very interesting history. It is not merely an exposure across a simple Tertiary monocline (visiting White Nothe will show this). The monocline is not just faulted; it is associated with major unconformity on the north side. This unconformity at the base of the Gault is known as the Sub-Albian Unconformity or the Late Cimmerian Unconformity. Recent studies onshore and offshore with seismic and in relation to oil exploration have provided new and clearer informaion about this structure.

Advanced parties would probably discuss this and, of course, it is appropriate to oil industry groups (but it might be rather complicated for some school parties). The English Channel Inversion is the potential oil-generating basin to the south and southeast of Lulworth Cove and Durdle Door (for more on this see: Petroleum Geology, South of England, webpage. The northern boundary of the basin effectively passes through the Lulworth and Durdle Door area. It is not a narrow, clear-cut fault line, though and has a number of complications.

In the Jurassic, and especially in the later part, the basin to south of Lulworth was subsiding and acquiring thick accumulations of sediments. This basin was uplifted and this is termed "inversion" in the Cretaceous. Movement was evident by Purbeck times, but the most conspicuous effects occurred in Late Aptian times (Carstone). Thus the base of the Albian here represent an unconformity with overstep in places onto the uplifted sediments of the inversion basin to the south. At Durdle Door the overstep is present but not conspicuous to the casual observer.

Go to White Nothe (further west) to see the substantial overstep of the basal Gault onto the Kimmeridge Clay. The unconformity is immediately obvious, even to elementary groups. It is not at Durdle Door, but with study its effects can be detected. There is no Lower Greensand present; there is a drastic thinning of the Wealden from 706m (at Swanage) to 49m, there are reworked Late Jurassic pebbles in the basal Gault pebble bed (but you are unlikely to see this in the field, and the bed is not always exposed because of landslipping). There is also a 10 degree difference in dip (with overstep downwards) between the Wealden and the Upper Greensand.

All this is very familiar to local petroleum geologists. However the more subtle effects are not always noticed. Whereas the Tertiary structural margin (i.e. the monocline) is sharply defined and very obvious, the English Channel Inversion margin is not abrupt for the simple reason that it was extensional and not the result of compression. The thin Purbeck Group here and, in particular, the thin Caps and Broken Beds in the Lulworth area (contasting with the thicker Purbeck anhydrite offshore) are the result of a sloping margin to the Inversion basin (a complication is that the coast is oblique to the Inversion margin, so that Durlston Bay is in the Inversion, but Lulworth is only on the border; Worbarrow Bay is on the true boundary).

I have said little above the Tertiary compression (the "inversion") and faulted monocline, but that is much more well-known, and relatively simple to explain (it will be considered separately). If this discussion above has now been understood, the Durdle Door section can be now be appreciated in correct perspective. The labelled photograph provided here shows the boundary between strata that originated in the Inversion Basin and those that were deposited after the Sub-Albian Unconformity.

However, if you have just an elementary field party then it is simpler and quicker just to discuss the Tertiary movement. The Inversion discussion is essential for oil industry parties, though (it needs special diagrams and it can be considered at another locality such as Stair Hole or Lulworth Cove).

LOCATION - DURDLE COVE:

Gault and Upper Greensand

Next above, stratigraphically, (i.e. northward) comes dark grey, Gault Clay of the Albian Stage of the Cretaceous. It usually causes landslides and, as at other localities, there some collapse of the cliff. At the base of the Gault is a pebble bed which marks the transgression of the sea over the Wealden fluvial environments. The Gault can contain marine fossils but they are difficult to find unless the exposure is clean and unweathered. The Gault becomes more silty and sandy upwards and passes up into the Upper Greensand in a transitional manner.

Northward of the Gault, the west side of the Durdle Door peninsula has a good exposure of the Upper Greensand. One bed, as shown above, contains conspicuous Thalassinoides ichnofossil (burrows of decapods or crabs) cemented by calcite and protruding from the base of the bed. The hard projecting Exogyra Bed with Exogyra conica and Rotularia concava ( Serpula concava ) is easily recognised. The Chert Beds are also easily seen, although the chert is not black like the Portland chert but usually brown or grey. Sponge-spicules within it, with the aid of a hand-lens. Some of the Upper Greensand chert follows burrows. Notice the boulder bed just beneath the basement bed of the Chalk. The latter has small brown phosphate nodules, some replacing fossils such as small ammonites. Phosphatised ammonites can also been seen in the phosphate beds of the Upper Greensand. Look for a recess in the cliff in the Upper Greensand section and search for small brown or black phosphatised fossils in the softer more argillaceous units.

LOCATION:

Durdle Cove - UGS - Chalk Junction

LOCATION:

Durdle Cove - Chalk Stratigraphy and Fossils

The Chalk of the Durdle Cove area is Lower Chalk, Middle Chalk and the lowest part of the Upper Chalk. The old terminology was in terms of the classic Chalk zones - Schloenbachia varians, Holaster subglobosus, Inoceramus labiatus, Terebratulina lata, Sternotaxis planus (Holaster planus). etc, There had been available, however, in recent years and in addition a useful lithostratigraphic nomenclature of Mortimore and the British Geological Survey. If this terminology is used, some translation is needed. Thus a table is provided above to help with discussion of the Chalk.

It is necessary to have some knowledge of the fossils of the Chalk that are used for zoning. Thus some fossil illustrations of a few common Chalk fossils are given below.

Echinoids occur in the chalk here, in the Durdle Door area. Specimens of Micraster are present but are usually abraded by sea erosion and, thus, only an outline of the thin test is visible. Some examples of Cretaceous echinoids are shown in the illustration above.

LOCATION:

West of Durdle Door - Durdle Cove Chalk

Beyond, in a north-westward direction, is Lower Chalk of Cenomanian age. This is, as usual, cyclically banded but it is very sheared here. Next comes the conspicuous unit of marl - the Plenus Marl, which is also very sheared with conjugate shears. It is well-exposed in a recess as shown in a photograph above. Middle Chalk follows in the younging direction, to the north, and it is mainly nodular and without flints.

LOCATION:

Durdle Cove - Plenus Marl and Sheared Chalk

Vertical Chalk and Upper Greensand in Durdle Cove is shown here in a fairly recent photograph and in an old photograph of Rowe and Sherborn (1902). Notice the plenus Marl in both photographs. The bearded gentleman in the old photograph (who is it - Dr Arthur Rowe?) with the black leather gaiters and stick is pointing to the Chalk Rock of the Geological Survey. This is a band of yellowish nodules, a conglomeratic horizon, usually glauconitic but here oxidised to a yellowish colour. Note the uppermost bed of the Upper Greensand forming a prominant wall, and to the left (stratigraphically above) the phosphatic basement bed of the Chalk. Note that the Lower Chalk has been placed by Rowe entirely in the subglobosus zone. The plenus marl at the top of the Lower Chalk has been displaced northwards by a small thrust fault.

LOCATION:

Durdle Cove - Thrust Plane in the Chalk

Chalk cliffs west of Durdle Door. The cliffs mainly consist of vertical Middle Chalk with some flints. The thrust plane of Durdle Cove is conspicuous (Arkell, 1947, p. 289) and has some fault breccia above it. This is near the junction of Middle and Upper Chalk. The slide plane or shear plane (classified by Arkell, 1938, as belonging to group 3a) dips south at various angles between 0 and 20 degrees. The hanging wall (the part above the fault) have moved north by up to 4 metres (Strahan, 1898) and there is a fault breccia of up to a third of metre in thickness. The sea has eroded the fault plane into a series of caves. You can see that it does not continue west of the embayment where the dry valley of Scratchy Bottom comes down to the sea. It is replaced by a number of smaller shears.

An old photograph from the classic work of Rowe and Sherborn (1902) showing the Chalk zones west of Durdle Door. The whole succession is Upper Chalk (or Ramsgate Chalk), with as mentioned above, the junction with the Middle Chalk (Seaton Chalk) near beach level. Annotations are based on those on an overlay of Rowe with minor modifications. Abbreviations are for the following zones.

Coniacian (lower part of Senonian)

M.ca. = Micraster coranguinum
M.ct. = Micraster cortestudinarium

Turonian

H.p. = Holaster planus (although Turonian this is part of the Upper Chalk and contains flints. This chalk is greyish)
T.lata = Terebratulina lata
Inoceramus labiatus

The Inoceramus labiatus and Terebratulina lata zones total 21 metres in Durdle Cove. Note that an older zone occurs in the upper part of the cliff. This is because the strata are overturned to some extent. Note also that the Terebratulina gracilis zone in Rowe and Sherborn's work has been replaced in this image by the more modern Terebratulina lata zone. The brachiopod Terebratulina gracilis actually only occurs in the upper part of the much higher Belemnitella mucronata zone according to Arkell (1947). A similar correction has been applied elsewhere where Rowe and Sherborn's photographs are used. The Inoceramus labiatus zone is seen in the eastern corner of Durdle Cove and is not clearly visible in this image.

There is clear increase in vegetation on the Chalk cliffs since 1902. Further comment is made on this elsewhere, but it is particularly obvious from these two photographs.

LOCATION:

Durdle Cove and Beyond - More on Structural Features

Bevan (1985) has provided a well-reasoned explanation of this and the numerous other shears of these cliffs. Sigma one, the maximum principal compressive stress, is near horizontal and there has been a net elongation parallel to the vertical layer-dip. Put into plain language, this means chalk has been squeezed from south to north, thinned in this direction and, in contrast, stretched out vertically. It is not surprising that this has happened near the foresyncline (the sharp bend of the fold), which intersects the coast at the inaccessible beach west of Bats Head. The proximity of the Portland Stone to the Chalk (but there are also other factors) and the faulting-out of parts of the Purbeck succession at Durdle Door ties in with this (to understand structures here, though, it is important to visualise the plunge of the monocline down to the east and to realise that what is seen near Durdle Door represents a much lower part of the fold in the Chalk than at Swanage or the Isle of Wight, and this lower part has a sharp bend)

LOCATION:

Scratchy Bottom

Scratchy Bottom is a dry valley in the Chalk, that descends southward to cliffs where there is an embayment about 300m. west of Durdle Door. The mp reference is SY802803. We are concerned here with the embayment in Chalk at the southern end.

There is a impression that much erosion has occurred at this southeast corner of the Scratchy Bottom embayment when seen on the 23rd March 2012. However, this has not yet been confirmed by comparison with similar photographs from the same locality.

At the small embayment at the foot of valley of Scratchy Bottom, a place where there is usually a rough path up the cliff, it can be seen that solifluction has moved the upper part of the vertical chalk. On the east side this movement appears to be southward. Material has slid down the valley in the periglacial conditions of the Pleistocene. Brownish chalky hill-wash or head is clearly seen on the west side of the embayment. Mammoth teeth occur in similar material in a similar site at Freshwater Bay on Chalk in the Isle of Wight, but they seem not to have been found here, as yet.

Rowe (1902) studied the Chalk on the east side of the Scratchy Bottom. His photograph with his annotations (modified) is reproduced here by kind permission of the Geologists Association. The embayment shows overturned Upper Chalk of the Holaster planus zone, the Micraster cortestudinarium zone (Lewes Nodular Chalk Formation in part) and the Micraster coranguinum zone (mostly Seaford Chalk Formation). A band of yellow nodules separates the two Micraster zones. The Micraster cortestudinarium zone here consists of yellowish nodular chalk. This zone is 21 metres thick and fossiliferous. The zonal Micraster has been found. The Micraster coranguinum zone is not very fossiliferous but part of an ammonite of the leptophyllus group was found by Rowe at the head of the recess. This was quite an unusual discovery.

The slide plane shown here, effectively a small thrust to the north, is the same one that forms the line of caves in Durdle Cove.

For returning to the cliff top there is an easy, but steep, little path running up a gully in the back of the Scratchy Bottom embayment was often used (I have used it for more than half a century). Do not necessarily always rely on this as a return route because now though (2012). A small but intentionally discouraging, wooden barrier has been placed at the top (probably for the usual health and safety reasons). A party, would probably find it easier to return to the steps at the Durdle Door peninsula.

LOCATION:

Swyre Head - High Chalk Cliff (west of Durdle Door)

.

.

.

[Safety - avoid the base of the cliff and watch out for risk of falling rock! The cliff top is also hazardous.]

With a summit at 98 metres in height, Swyre Head is the highest vertical Chalk cliff in Dorset (Hambury Tout is higher but not vertical). It has some superficial resemblance to the White Cliffs of Dover. Swyre Head and adjacent Chalk cliffs in the Bat's Head area have been used in films from time. On one occasion I found no access to the beach here, and the nearby car park at the Durdle Door camp site contained vehicles with filming equipment. It is quite likely that the Chalk cliffs around here were used by the military for practice for the Normandy Invasion in the Second World War (because of similar Chalk cliffs in Normandy).

There are good geological and geomorphological reasons for the height and steepness of the cliff. The cliff face is almost vertical mainly because the bedding is vertical within the middle limb of the Tertiary monoclinal fold. Erosion by an abrasive shingle beach is active at the foot of the cliff (although there seems to be no notch), and the cliff has no protection from southwesterly storms. As might be expected with the severe Tertiary folding, there has been much bed-over-bed slip in a vertical direction. Thus the bedding planes visible in the cliff face can be seen to have well-developed slickensides.

Although Swyre Head presents a fine steep cliff about 90 metres high, the Chalk hill summits to the west are rather higher, and are probably related to a peneplain at about 150 - 200 metres.

The two dry valleys between Durdle Door and Bat's Head may at one time have ended in coastal embayments, rather like the present Lulworth Cove (Jones et al. , 1984). Note, however, that Lulworth is somewhat unusual for having an active stream in the valley. Old coves here, though, have now coalesced and now there is a stretch of open water between Durdle Door and Bats Head, and a fairly straight coastline between these surviving headland.

LOCATION SWYRE HEAD:

Flint Nodules

The details of the flint bands in the vertical micraster coranguinum Zone Chalk can be seen very easily and conveniently on well-exposed surfaces at the foot of the cliff. Some bands consist of almost linking, roughly ovoid flint nodules. Others are in the form of bands of scattered small flint nodules.

LOCATION - SWYRE HEAD

Faults and Slickensides

Bedding-parallel fault planes with slickensides are particularly well-developed in the near-vertical Micraster coranguinum Zone Chalk at Swyre Head. In March 2012 it was noticed that there had recently been several cliff falls related to these planes of weakness.

LOCATION:

Swyre Head - Erosional Changes with Time

In the photograph above, Swyre Head is shown in recent years on the left and in 1902 on the right. This comparison is provided to show changes in the amount of vegetation on the cliff. It is obvious by comparing other old photographs with modern ones for this area that much Chalk has become covered in grass since 1902. If the rate of coast erosion has been constant then roughly the same amount of vegetation would be expected on the cliffs. Perhaps the the cliffs have stabilised to some extent. It is suprising that coast erosion should reduce if the sea-level is rising more rapidly now than in the past. (A later note - March, 2012, erosion at Swyre Head seems to be more active again.)

Possibly the cliffs retreated drastically at some time in the past, perhaps in the great storm of November 1703 with its south-westerly hurricane winds and the exceptionally high tides (Defoe, 1705). Clean white Chalk cliffs might have been produced then by a phase of intense erosion. If erosion has been more moderate since that date the consequent stability could have favoured the growth of grass. However, the matter has not been investigated and it is possible, indeed likely, that the increased amount of vegetation is the consequence of global warming. (See also details regarding the Chalk cliffs of Lulworth Cove, which have also become vegetated.)

LOCATION:

Swyre Head - Fossil Content

The cliffs west of Scratchy Bottom consist of the Micraster zones of the Upper Chalk (Coniacian - Micraster cortestudinarium , below, and Micraster coranguinum above. Micraster is the common heart sea-urchin of the Chalk, a creature which to some extent burrowed in the white mud of the sea-floor in late Cretaceous times. Good specimens are not easily found here, largely because the chalk is harder here than in most other areas because of local tectonism. Thus they do not break out easily but are cut through by erosion. Rather than look for the surface appearance of an echinoid, look for an oval cross-section with a thin shell. Examples are common. (In contrast, at Southampton University there were at one time very large numbers of good specimens of Micraster coranguinum in the Westlake Collection from the Micheldever (Hampshire) railway cuttings in the 1890s.).

LOCATION:

Swyre Head - Cliff and Platform of Marine Erosion

Swyre Head once projected further south and separated two bays. It has been eroded back so that this stretch of coast has been almost amalgamated into one bay. In the course of this the foot of the headland has been converted into a platform of marine erosion. The cliff is still being cut back with the consequence of falls of chalk debris from time to time. The Chalk is vertical here and has been cut across cleanly by marine erosion, revealing fossils and sedimentary structure in cross-section. The photograph was taken in April 2002 and this piece of coast is likely to show further changes in the future. Examine the old photograph of Rowe and observe changes at the foot of the cliff.

In this view to the southeast, the prominant offshore rock is the Bull, of Portland Stone. Not shown and directly to the south is a smaller, largely submerged Portland Stone rock, the Blind Cow which was part of an outcrop once affording some major protection to Swyre Head. Durdle Door promontory is in the distance.

LOCATION:

Bat's Head and Butter Rock

Bat's Head is a conspicuous promontory, mainly of Micraster Chalk (but with some Sternotaxis planus Chalk at the extremity. The promontory is very narrow and, as can be seen a small cave or natural arch cuts through it. Eventually a new stack will be formed after erosion has increased and led to collapse of the arch.

Adjacent to Bat's Head, on the east side, is a small chalk stack or pinnacle. This is aligned south of the small dry valley between Bats Head and Swyre Head. The bedding is vertical as shown by the lines of flints.

The offshore Portland Stone rock, the Cow, may be seen in some photographs. It is part of a submerged ridge of Portland and Purbeck stone.

LOCATION:

Bats Head - More

For more information on Bat's Head please go to the:

White Nothe to Bat's Head Webpage - in preparation.

LOCATIONS:

Chalk Cliffs from Bat's Head to White Nothe

The beach beyond Bats Head is not usually accessible except by boat but, as mentioned, is of particular interest in showing the foresyncline. You can, as I have done in the past, swum round Bats Head wearing geology boots and carrying a geology hammer, but this is not now recommended for safety reasons!

Please go to the: White Nothe to Bats Head geological field guide. (a new guide in preparation)

MISCELLANEOUS:

Dolphin at Durdle Door - 2002

On the 12 August, 2002, on the landward side of the natural arch of Durdle, tourists were swimming with a dolphin. The friendly dolphin, probably the one known as George, was spending hours playing with the children and adults in the water. A few pictures are added here for interest. At the time of his appearance at Durdle Door, the weather was warm (for England) and the water clean, clear and calm. He attracted many people to the beach and came into water quite shallow enough for children to wade in. Periodically he swum out towards the natural arch.

MISCELLANEOUS

Chalk Downs at Durdle Door - Soil and Vegetation

The short grass is usually grazed and grows on a thin calcareous soil. Water percolates underground through the chalk so the surface is usually relatively dry. Harebells, seen here in August, 2002, just northwest of the Door, are small blue flowers with joined petals and five lobes. The species is Campanula rotundifolia it is a member of the campanulaceae.

REFERENCES

Please go to the separate Lulworth Bibliography.

ACKNOWLEDGEMENTS

I thank the various students of Southampton University and London South Bank University for participating in field trips to Durdle and allowing photography for the internet. I am very grateful to the Geologists' Association to reproduce the photographs of Rowe. Dr Clive Needham and the Staff of Agip UK Oil Company are thanked for digital photographs of the area. Discussion with the late Professor Michael House has been very helpful. I thank Halfdan Carsten for his kind permission to use photographs taken on a field trip in Dorset in 2005. I thank Alan Holiday for the photograph of Durdle Door from the sea and for photographs of the cliffs from the shore. My wife Cathy has very kindly provided background support and, in addition, cameras for the production of this and all the other webpages. I much appreciate the advice and help of my daughter, Tonya Loades of Bartley West, Chartered Surveyors.